Jun 7, 2019 in Science

Geology Assignment

Introduction

Geology is a complex of sciences dealing with the hard shell of the Earth, of the history of its development, and the processes that created it. The current field of science investigates the rock formations and helps understand how and when they were formed. The geological structure of Great Britain is very diverse and has been studied by scientists for a long time. There are large basins of gneiss, granite, chalk, and red sandstone of different geological times. The knowledge of their time of formation is very important in construction. Thus, if the foundation is being built on the peat, it is important to know its depth of bedding not to cause the foundation subsidence. Moreover, hydrogeology studies the environmental aspects of exploration, search, and formation of groundwater. The knowledge of their location helps prevent establishing of the industrial plants, agricultural fields, and waste disposals areas that contaminate aquifers near them. Thus, the current paper explores the most important rock formations of Great Britain, the ways of aquifers contamination and the problems of the foundation subsidence on the peat. Moreover, the paper provides information, which can help understand the geological environment better, outlines recommendations on how not to contaminate it, and how to build on the weak soil.

Rock Formations in Britain

Gneiss is a granular metamorphic rock formation. It primarily consists of feldspar, quartz, and coloured minerals. By the nature of primary rocks (degree of metamorphism), the scientists distinguish two types of gneiss, namely para-gneisses and ortho-gneisses. The first type is formed due to deep metamorphism of sedimentary rocks, and the second one is due to magmatic (mainly volcanic) processes. Gneisses are the oldest rocks in Britain. They are found in Hebrides array (protrusions of the Precambrian North-West Scotland and the Hebrides). They were formed 2,7 billion years ago in the Archean. Gneisses are also found in Precambrian craton of Wales and the Midland that is composed of the complex of Upper Precambrian gneisses and crystalline schists. There are also Lewisian gneisses found in the north-west Scotland. Parts of them are thought to be almost 3 billion years old (McKirdy et al. 2007). 

Granite is an intrusive rock formation of acid composition having the granular structure. There is a pure granite (10-65% plagioclase), granodiorite (65-90%), tonalite and trondhjemite. Ancient sedimentary rocks of the late Paleozoic era with huge granite cores are situated in the rolling hills of Devon and Cornwall, the south-western peninsula of England. The moorlands, such as Dartmoor and Exmoor, are confined to such cores. Breeds that clothe the granite intrusions are partially mineralized. In addition, in the north of the British Islands, within the area of Caledonian folding, crystalline rocks of Archean and Proterozoic eras with granite intrusions are found (Erickson & Muller 2001). 

 

Chalk is a sedimentary semi-coherent smear little-cemented carbonate rock formation that is represented by 90-99% calcite, which consists of calcite residues of marine planktonic algae and small particles of simple organisms’ shells (Toghill 2000). In the Cretaceous period, the large part of Britain was below the sea level, which has led to the accumulation of the chalk on its territory. A fine-grained white chalk, formed at that time, composes sharply distinguished and almost solid ledge that forms a huge arc from Cape Flamborough Head on the east coast of Humberside to Cape Portland Bill on the south coast of Dorset. In the south of Wiltshire (where Stonehenge is situated) to the north of Salisbury, Cretaceous zone expands to Salisbury Plain. Moreover, two narrow chalk hills, namely North Downs and South Downs, stretch to the east around the complex geological structure of the hill Te Weald and reach the sea in the form of "white cliffs of Dover" and Cape Beachy Head. Cretaceous rocks of the Cretaceous period are also situated in the London Basin and the Hampshire Basin (Toghill 2000). 

The red sandstone is named due to its red colour caused by the presence of iron oxides (hematite). Red Sandstone of Great Britain is presented by Old Red Sandstone and New Red Sandstone. The ORS was formed in Devonian Age mostly in Scotland. The ORS formations are situated in the Orcadian Basin, in the Midland Valley of Scotland, in the Scottish Borders, in the Anglo-Welsh Basin, in Pembrokeshire, and in Anglesey. The New Red Sandstone is a British geological term for the stratums of red sandstone, which associates rocks that formed during the Permian (280 million years ago) to the end of the Triassic period (near 200 million years ago) (Woodcock & Strachan 2012). The formations of NRS are mostly situated in the central England, where they form a low-lying plain except the Mid Severn Sandstone Plateau. Thick stratums (up to 1100 m thick) are found in the damaged Cheshire Basin that expands beneath North Shropshire. The NRS is also situated in Lancashire and Cumbria, and in the east of the Pennines it expands to the areas of Nottinghamshire and central Yorkshire. Additionally, geologists found the formations of NRS in the Red Cliffs of Dawlish and East Devon (Miller 2010). 

Contamination of the Aquifers

The majority of cases of the aquifers contamination occur due to the industrial activities. Industrial plants pollute the aquifers through the effluents that flow into the water. Various inorganic (acids, salts, alkalis) and organic (oil and its products, organic surface substances and surfactants, synthetic detergents, etc.) harmful impurities may enter the water with effluents. Most of the contaminants are toxic to the inhabitants of the water and for the man, as well. The organic products have the detrimental effect on the water bodies’ conditions, as they lower oxygen content in the water. Oil and oil products are especially harmful in this regard as they form a pellicle on the water surface that prevents a gas exchange. A large number of organic pollutants are discharged into the water by pulp and paper mills, oil refineries, and chemical plants. Another way of industrial contamination is physical. Sand, silt, and clay enter the water from washing installations of mining enterprises, with the emissions of radioactive impurities by NPPs and from thermal power plants with particles of ash. In addition, contamination can occur due to the discharging of warm waters from different energy installations of NPP and TPP to the water bodies. NPPs usually discharge water of 45 °C, which is disastrous to the water inhabitants. To monitor the aquifers pollution, the industrial plants must establish their own cleaning systems that will clear their effluents. Thus, the harmful substances will not enter the aquifers.

Waste disposals can also contaminate the aquifers. Such garbage as plastic, oils, pesticides, and solvents that allocate harmful substances during the decomposition mix with the water and pollute it. Additionally, such electronic garbage as lamps, batteries, and switches, can cause the mercury and cadmium pollution of the water. Another way of contamination is a methane formation due to the anaerobic processes that occur in the buried layers of garbage without air. A fundamental solution to the problem of garbage is a recycling of waste. It will lessen the amount of waste and, as a result, will lessen the amount of harmful substances. 

Agricultural activities can pollute the aquifers, as well. Frequently, the agricultural fields are fertilized by hazardous substances, such as pesticides. They all enter the aquifers with effluents. They can also be contaminated physically. Sand, silt, and clay can enter the water through surface wash-off of the farmland by rain water. To monitor the contamination, the hazardous substances must be prohibited to use on the agricultural fields. Unfortunately, the physical contamination cannot be managed.

The Subsidence Hazards Associated with Foundations on Peat

Peat is a mixture of sand and clay soils with plant residues. It has a large compressibility and anisotropy. In addition, the peat may contain the elements that aggressively affect materials of construction’s underground structures. Consequently, peat is considered a very weak soil. Peat layer provides significant subsidence for a long time (Huat et al. 2014).

Thus, at subzero temperatures groundwater turn into ice, and, therefore, increase in volume (approximately 10%), which is a main cause of soil swelling. During this natural process, apart from the compressive loadings, tangentially efforts begin to act on the foundation pushing it up. The size of the loading is directly proportional to the depth of freezing. Sometimes it can be measured to 12 kN for each square meter. And if the soil freezes below the depth of the "sole", its efforts are added to the lifting force acting on the sole. Such value is already greater than the sum of side forces of outthrust. Naturally, the foundation and, therefore, a house built on it, may be in a state of rest only when the forces acting on it balance each other in two directions (upward and downward). If the swelling forces are too great, then the rise of the foundation is inevitable. After thawing of the soil, "sole" can return to the starting position or remain at the same level. In the worst case, –it can return to its previous point unevenly. In the latter case, the deformation of the foundation, as well as the whole structure, can take place (Bell 2004).

To avoid such unfavourable effects of frost swelling forces on the foundation, a number of serious constructive measures must be planned and performed. Firstly, it is important to prepare the basis of the "sole" below the level of the soil freezing. It will prevent its negative lifting force acting from below on the “sole” construction. Secondly, it is recommended to cast the foundation walls of reinforced concrete. Thirdly, it is necessary to expand the basis of the "sole", which will act as a sort of "anchor" and make the foundation walls vertical and slightly narrowing upward (in the form of a trapezoid). In such case, during compression of the walls by the soil, the lifting capacity decreases. The next step is to cover the walls of the "basis" with a sliding material (roofing felt or similar to it), that is a lubricant. However, the best way to solve the problem is to use a floating foundation. It is a solid plate that allows to evenly distribute the load acting on the "sole" both from the soil and from the building. It is placed at a reasonable depth. In addition, to reduce the swelling forces, the soil beneath is replaced with sand that creates a sand cushion. Such method is aimed at neutralizing the soil force.

Conclusion

In conclusion, the rock formations of gneisses are the oldest rocks in Britain. They were formed 2,7 billion years ago in the Archean period. Moreover, the gneisses were also formed in Precambrian age. The rock formations of granite are of the late Paleozoic and of Archean and Proterozoic periods, while the rock formations of chalk are of the Cretaceous period. Red sandstone of Great Britain is presented by Old Red Sandstone and New Red Sandstone. The ORS was formed in Devonian age. The NRS was formed during the Permian to the end of the Triassic periods.

Industrial plants pollute the aquifers through the effluents that flow into the water. The industrial contamination can also be physical due to the sand, silt, and clay pollution, as well as radioactive impurities, and ash. In addition, contamination can be caused by the descent of warm waters of NPP and TPP. Waste disposals can contaminate the aquifers through allocation of harmful substances during the decomposition. The agricultural fields pollute aquifers through effluents and physical processes. To monitor the aquifers pollution a number of measures have to be taken. They include establishing of cleaning systems on plants, recycling of waste, and prohibiting to use hazardous substances.

Peat has a large compressibility and anisotropy and contains the elements that negatively affect materials of construction’s underground structures. It is a very weak soil that provides significant subsidence for a long time. The best solution to overcome such problem is to use a floating foundation when constructing a building on the peat.

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